A Micro-Electro-Mechanical Systems (MEMS) device is typically enclosed in a sensor block assembly (SBA) to facilitate mounting of the MEMS device in its application device. For example, a MEMS gyroscope or accelerometer may be used to determine angular rotation or acceleration, respectively, of the application device in which it is installed.
The MEMS device may be subject to severe temperature excursions when the application device is operating in the field. For example, if the application device is a satellite or space craft, the satellite or space craft will be subjected to extreme temperature variations. However, performance of the MEMS device varies as a function of temperature. It is particularly undesirable for the MEMS device to be subjected to changes in temperature.
Further, in addition to ambient temperature excursions, devices within the MEMS device and the SBA, including the SBA controller, generate heat while operating due to resistive loading losses. Such generated heat further complicates the maintenance of the MEMS device at a predefined temperature or within a predefined temperature range.
To prevent the MEMS device from exposure to cold temperatures, a heater may be physically coupled to, or implemented within, the sensor block assembly. As temperature of the application device decreases, the SBA heater will operate to maintain temperature of the MEMS device within a predefined temperature threshold.
However, the temperature control system of the SBA heater relies on an external power source, such as a battery. When a battery is used as the power source, output voltage of the battery changes as a function of temperature and/or use. For example, as power within the battery is consumed, the output voltage of the battery will decrease. As the battery is recharged, output voltage increases. Such changes in the output voltage of the battery affects the performance of the SBA heater.
The output power of a resistive type SBA heater equals the magnitude of the input voltage squared (VIN2), divided by the value of the heater resistor (RHEATER), and times the duty factor (D), as illustrated by Equation 1 below. The duty factor corresponds to the period of time for which the input voltage (VIN) is applied to the heater resistor (RHEATER).Output Power=[(VIN2)/(RHEATER)]*(D)  (Eq. 1)
The SBA heater control system is configured to adjust the duty cycle so that the SBA heater maintains the operating temperature of the MEMS device at its predefined operating temperature (or within its predefined operating temperature range). However, the SBA heater control system response time is not instantaneous. For example, if the power supply voltage doubles, the output power increases by a factor of four until the SBA heater control system is able to decrease the duty cycle by a factor of four. During this period of time wherein the SBA heater control system adjusts the duty cycle, the SBA heater may heat the MEMS device above the desired predefined temperature (or temperature slew rate).
Accordingly, it is desirable to improve the ability of the SBA heater control system to respond more accurately to changes in the power source voltage so that the SBA heater is able to maintain the predefined temperature of the MEMS device.